1. Field of the Invention
The present invention relates to a hydraulic pressure control valve for use in a hydraulic brake system of a vehicle.
2. Description of the Prior Art
In a vehicle's hydraulic brake system, a tandem master-cylinder is used as a master cylinder, and in this type of master cylinder, the hydraulic system is separated to two independent hydraulic lines, one being for the front hydraulic line for the front wheels, the other for the rear hydraulic line for the rear wheels. In the arrangement of the hydraulic brake system having such two independent hydraulic lines therein, there is provided a hydraulic pressure control valve adapted to effect the pressure relieving function on the rear hydraulic line, in order to provide a stable braking operation when a vehicle is braked, in coping with a trend of gravity center's movement toward the running direction of a vehicle due to the inertia thereof.
As one typical example of this type hydraulic pressure control valve for use in a vehicle's hydraulic brake system, there is generally known a load-responsive type valve. This load-responsive type valve comprises an inlet port connected to the master cylinder, a pressure relieving mechanism adapted to effect a pressure relieving function on the oil pressure from the master cylinder at a constant rate depending upon the load of a vehicle, and an outlet port connected to the slave or wheel brake cylinders in the rear hyraulic line and adapted to direct thus relieved hydraulic pressure thereto. According to the conventional load-responsive valve, the operating pressure of the hydraulic oil served in the rear hydraulic line is caused to be relieved or reduced depending upon the load of a vehicle, thereby aiming to assure a stable braking performance.
However, should there be any damage such as an accidental leakage in the front hydraulic line, it is inevitable that the braking effect from the hydraulic brake system of a vehicle has to depend only on the rear hydraulic line. In such an occasion, it is not preferred that the pressure relieving mechanism operates.
In order to cope with such problem, there have been proposed various bypass mechanisms for the purpose of releasing or cancelling the function of such pressure-relieving mechanism as stated above, such bypass arrangement would obviously add the complexity of the conventional load-responsive valve systems when incorporated as a unit therein. As the load-responsive valve is usually arranged on the rear axle side, an extensive hydraulic piping is required which leads the hydraulic oil in the front hydraulic line to the bypass mechanism of the load-responsive valve for the bypass mechanism.